This subject has been debated for years. In SC according to the Palmetto Gun Club SLED told them that CC in a Post Office is OK as long as you don't go behind the counter. See half way down on page 6 under CWP COURSE NEWS/NRA COACH SCHOOL http://www.palmettogunclub.org/nwslt...09Bullseye.pdf There is still no clear cut ruling on this.
See Code of Federal Regulations Title 36, Chapter 1, Section 232.1
(l) Weapons and explosives . Notwithstanding the provisions of any other law, rule or regulation, no person while on postal property may carry firearms, other dangerous or deadly weapons, or explosives, either openly or concealed, or store the same on postal property, except for official purposes.
(p) Penalties and other law. (1) Alleged violations of these rules and regulations are heard, and the penalties prescribed herein are imposed, either in a Federal district court or by a Federal magistrate in accordance with applicable court rules. Questions regarding such rules should be directed to the regional counsel for the region involved.
(2) Whoever shall be found guilty of violating the rules and regulations in this section while on property under the charge and control of the Postal Service is subject to fine of not more than $50 or imprisonment of not more than 30 days, or both. Nothing contained in these rules and regulations shall be construed to abrogate any other Federal laws or regulations of any State and local laws and regulations applicable to any area in which the property is situated.